OGT Level Manager
Lock and unlock levels easily with OGT Level Manager for Corona SDK.
Super Sprite Bundle
Royalty-free graphics for 2D games - animated characters, tile sets, etc.
GameDevMarket.net
Great quality (but budget priced) 2D and 3D artwork you can use in your own games.
Pathfinding Tutorial
Use Corona SDK and the free Jumper library to do A* pathfinding in your games - over 60 minutes of video + sample code!
KoboldTouch
An easier/better way to use Cocos2D for 2D game development.